Shoot biomass and zinc/cadmium uptake for hyperaccumulator and non-accumulator Thlaspi species in response to growth on a zincdeficient calcareous soil

Thlaspi caerulescens is one of the best-known heavy metal hyperaccumulating plant species. It exhibits the ability to extract metals from soils and accumulates them in shoots at extremely high concentrations, particularly zinc (Zn) and cadmium (Cd). Using T caerulescens (J. and C. Presl, ecotype Prayon) and a closely related non-accumulator species T arvense, greenhouse experiments were carried out to study shoot growth (dry matter production) and Zn and Cd uptake from a severely Zn-deficient calcareous soil (DTPA-Zn: 0.09 mg kg(-1) soil) supplemented with increasing amounts of Zn (0, 0.05, 0.5, 5, 25 and 75 mg kg(-1) soil) and Cd (0 and 25 mg kg(-1) soil). Shoot dry matter production of T caerulescens was severely depressed by Zn deficiency, while in T arvense, Zn deficiency slightly reduced growth. At the lowest Zn supplies (0 and 0.05 mg Zn kg(-1) soil), T caerulescens showed very severe Zn deficiency symptoms, including decreased leaf size and development of chlorosis and whitish-brown necrosis on the younger leaves. These symptoms were slight in T arvense. At the highest Zn supply, leaves of T caerulescens did not show any symptoms, but in T. arvense there were some necrotic patches on the margins of older leaves, probably due to Zn toxicity. With increasing Zn supply from 0 to 75 mg kg(-1) soil, shoot dry matter production was increased by 4-fold in T caerulescens and only 1.3-fold in T arvense. Supply of Cd resulted in marked decrease in shoot growth of T. arvense, particularly under low Zn supply, but had no effect on the growth of T caerulescens. At the low soil Zn levels ( < 0.5 mg Zn kg(-1)) shoot Zn concentrations were lower in T caerulescens compared with T arvense, and were below 10 mg Zn kg(-1) dry weight. However, at the high supplies of Zn ( > 5 mg Zn kg(-1)), shoot Zn concentrations were considerably higher in T. caerulescens than T arvense. Increase in Zn supply from 0 to 75 mg kg(-1) enhanced shoot Zn concentrations by 84-fold in T caerulescens and only 8-fold in T arvense. Shoot Zn concentrations of both species were not affected by Cd supply, while increase in Zn supply did not affect Cd concentrations in shoot of T caerulescens, but markedly reduced them in T. arvense. The results demonstrate that T. caerulescens is extremely sensitive to Zn deficiency in soils, but tolerant to excessive accumulation of Zn and also Cd in shoot, while T arvense is tolerant to Zn deficiency but not to accumulation of Zn and Cd in shoot. Hyperaccumulation of Zn in T. caerulescens possibly depends on the existence of high concentrations of plant-available Zn in soils, which suggests that root-based mechanisms associated with increasing metal availability in the rhizosphere (e.g., rhizosphere acidification or release of Zn-mobilizing organic compounds from roots) only play a minor role in metal hyperaccumulation by T caerulescens. The findings also suggest that the processes causing the metal hyperaccumulation trait in T caerulescens also cause this plant species to be sensitive to Zn deficiency stress. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.